When things started turning
dusk outside, he showed me all the places he and Brad would meet.

The setting of dusk is significant because it emphasizes the secretive nature of Brad and Patrick's relationship. Dusk is also a transitional time between day and night, and this could symbolize the change between Brad's public and private behavior towards Patrick.

I’m not sure if I will have the time to write any more letters because I might be too busy trying to “
participate.”

Although the readers would not see how this decision plays out, if followed, it would develop Charlie's character because he'd be more involved in living his life with others, but this would also make him less of a writer, because he'd no longer enjoy the perks of being a wallflower that would push him to the distance necessary to observe, reflect, and record.